Hughston, L P; Penrose, R; Sommers, P and Walker, M. 1972. On a Quadratic First Integral for the Charged Particle Orbits in the Charged Kerr Solution. Communications in Mathematical Physics, 27(4), pp. 303-308. ISSN 0010-3616 [Article]

Hughston, L P. 1986. Entropy, Uncertainty, and Nonlinearity. In: C J Isham and R Penrose, eds. Quantum Concepts in Space and Time. Oxford: Oxford University Press, pp. 174-181. ISBN 0198519729 [Book Section]